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Molecular orbitals

Molecular orbital diagram of B₂

Bond order 1, paramagnetic. 10 electrons: Nb = 6, Na = 4.

BB
  • B

B₂Boron

Bond order 1

10 electrons, 2 unpaired

i

B₂ is paramagnetic, which shows that π2p lies below σ2pz in B₂.

  • Bond order1
  • Magnetismparamagnetic
  • Electrons10

Molecular orbitals

10 electrons
1bond order
paramagnetic

(Nb − Na) / 2 = (6 − 4) / 2

1s1s2s2s2p2pσ1sσ*1sσ2sσ*2sπ2pσ2pzπ*2pσ*2pzB atomMoleculeB atomEnergy

σ1s² σ*1s² σ2s² σ*2s² (π2px¹ = π2py¹)

Working

Filling the diagram

  1. B₂ has 10 electrons: 5 from each B.
  2. Up to N₂ (and its ions), the π2p pair lies below σ2pz and fills first.
  3. Configuration: σ1s² σ*1s² σ2s² σ*2s² (π2px¹ = π2py¹)
  4. Bonding electrons Nb = 6, antibonding electrons Na = 4.
  5. Bond order = (Nb − Na) / 2 = (6 − 4) / 2 = 1.
  6. Two electrons are unpaired (in π2px and π2py), so B₂ is paramagnetic.

Class 11, Chapter 4 Chemical Bonding and Molecular Structure.

Questions

MO diagram of B2: quick answers

What is the molecular orbital configuration of B2?

σ1s² σ*1s² σ2s² σ*2s² (π2px¹ = π2py¹). Up to N₂ (and its ions), the π2p pair lies below σ2pz and fills first.

What is the bond order of B2 from its MO diagram?

1: Nb = 6 and Na = 4, so (6 - 4) / 2 = 1. It is paramagnetic.

Why is σ2pz above π2p in B2?

Up to N2 the 2s and 2p orbitals are close in energy and mix, which pushes σ2pz above the π2p pair.

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